1. Field of the Invention
The present invention relates in general to an electron microscope of a scanning type and more particularly concerns an improvement in a scanning type electron microscope which is provided with two specimen stages so that a high resolution image of a specimen having a small size and a specimen of a large size can be interchangeably observed.
2. Description of the Prior Art
In general, in contrast to a beam transmitting type electron microscope, it is required in the case of the scanning type that a specimen to be examined can be displaced or moved over a large range to thereby allow the whole surface of a specimen to be observed even when the specimen is of a large size (e.g. 10 cm in diameter and 7.5 cm in thickness). In this connection, description will be made in some detail on hitherto known scanning type electron microscopes referring to FIGS. 1 and 2. As is shown in FIG. 1, in order to meet the requirement described above, a specimen stage 1 is disposed behind an objective lens 2, whereby a specimen may be displaced two-dimensionally for a great distance, tilted in a range of -10.degree. to 90.degree. and/or rotated for 360.degree.. However, movement of the specimen over such great range not only makes the specimen stage to be very susceptible to vibrations but also gives rise to drift of the specimen due to thermal expansion of the specimen stage involving eventually instability of the produced specimen image. Further, because of a large distance between the specimen and the objective lens 2, various aberrations become significant. A large volume of a specimen chamber 3 for accommodating specimens of large size makes it difficult to maintain a high vacuum level within the lens column and provides additional problems such as contamination (e.g. deposition of carbon on the specimen surface). Such being the circumstances, many difficulties are encountered in attaining a high resolution power. As an approach to solve these problems, it has been proposed to use in combination with a hitherto known transmissive type electron microscope a removably attached device for inserting the specimen between magnetic poles of the objective lens of the microscope. With such arrangement, it is however impossible to observe large size specimens. That is, the inherent characteristic feature of the scanning type electron microscope will be lost.
In view of the foregoing situation, there has been proposed a structure of a scanning type electron microscope of a structure such as shown in FIG. 2 for allowing a large size specimen to be observed in addition to observation of a small size specimen with a high resolution power. As can be seen from FIG. 2, there are provided a chamber 4 for accommodating a large size specimen and a chamber 5 for a small size specimen. This structure however suffers from various shortcomings described below due to the disposition of the large-size-specimen chamber 4 over the small-size-specimen chamber 5. That is:
(7) Since other devices such as specimen transfer mechanism, X-ray spectrometer or the like in addition to the specimen stage 1 have to be provided within the chamber 4 for the large size specimen, the center of gravity of the whole lens column is located at a higher position, involving structural or positional unstability as well as degraded resolution power due to susceptibility to adverse vibrations. PA1 (2) When a small size specimen is observed by using a specimen stage disposed within the chamber 5, the chamber 4 for the large size specimen which is not used at that time has to be evacuated to a desired vacuum level (about 10.sup.-5 Torr), which means that a lot of time is required for the evacuation. Further, it is difficult to maintain the small-size-specimen chamber at the desired vacuum level due to the fact that the out gasing from the inner walls or other parts of the large-size-specimen chamber will flow into the chamber for the small size specimen. PA1 (3) Because of the presence of a large aperture formed in a side wall of the large-size-specimen chamber 4 for inserting the specimen table 1 into the chamber, the electron beam is likely to be subjected to disturting influences of external magnetic field through the aperture. PA1 (4) Since the objective lens for observing the specimen of a small size is installed at the bottom portion of the lens column assembly, it is difficult to mount other lens below the objective lens 8, imposing limitations to the performances (e.g. contrast, resolution, sensitivity) in producing transmitting/scanning image (i.e. STEM image), electron diffraction image, energy loss image.